Do you mean that (a) the hex head is stripped out such that your tool turns without turning the bolt or that (b) the bolt turns but does not come out?
If (a) you need to drill the center of the hex head with a drill bit to remove the head. This will let you pull the damper rod out and use vice grips to remove the remaining threaded part.
If (b) then the damper rod is probably just spinning in the fork leg and you need to reinstall the spring or use the handle of a broomstick to hold it in place.
If (a) you need to drill the center of the hex head with a drill bit to remove the head. This will let you pull the damper rod out and use vice grips to remove the remaining threaded part.
If (b) then the damper rod is probably just spinning in the fork leg and you need to reinstall the spring or use the handle of a broomstick to hold it in place.